The present invention is directed toward an improved dehydrocyclization process where light paraffinic hydrocarbons are converted with high selectivity to aromatics. More particularly, the activity of a nonacidic L-zeolite containing dehydrocyclization catalyst is enhanced by including water, water precursors, or mixtures thereof in a reaction zone with a C.sub.6 -C.sub.10 hydrocarbon feedstock.
In the past, it has become the practice to effect conversion of aliphatic hydrocarbons to aromatics by means of the well-known catalytic reforming process. In catalytic reforming, a hydrocarbonaceous feedstock, typically a petroleum naphtha fraction, is contacted with a Group VIII-containing catalytic composite to produce a product reformate of increased aromatics content. The naphtha fraction is typically a full boiling range fraction having an initial boiling point of from 10.degree. to 70.degree. C. and an end boiling point of from about 163.degree. to about 218.degree. C. Such a full boiling range naphtha contains significant amounts of C.sub.6 -plus naphthenic hydrocarbons. As is well known, these paraffinic and naphthenic hydrocarbons are converted to aromatics by means of multifarious reaction mechanisms. These mechanisms include dehydrogenation, dehydrocyclization, isomerization followed by dehydrogenation. Naphthenic hydrocarbons are converted to aromatics by dehydrogenation. Paraffinic hydrocarbons may be converted to the desired aromatics by dehydrocyclization and may also undergo isomerization. Accordingly then, it is apparent that the number of reactions taking place in a catalytic reforming zone are numerous and, therefore, the typical reforming catalyst must be capable of effecting numerous reactions to be considered usable in a commercially feasible reaction system.
Because of the complexity and number of reaction mechanisms ongoing in catalytic reforming, it has become a recent practice to attempt to develop highly specific catalysts tailored to convert only specific reaction species to aromatics. A disadvantage in processing C.sub.6 -C.sub.8 paraffins is that elevated temperatures are required for the reaction to proceed and the selectivity is directed toward undesired reactions such as hydrocracking. Until recently, traditional reforming catalyst compositions were not satisfactory for the conversion of light paraffinic hydrocarbons to aromatics. Today, catalyst compositions containing L-zeolite have been successfully used to selectively dehydrocyclize C.sub.6 -C.sub.8 paraffins to aromatics. As can be appreciated by those of ordinary skill in the art, increased production of aromatics is desirable. The increased aromatic content of gasolines, a result of lead phase-down, as well as demands in the petrochemical industry, makes C.sub.6 -C.sub.8 aromatics highly desirable products. However, the activity and activity-stability of these catalysts is well below what is needed for commercial processing of these light paraffinic hydrocarbons. It is, therefore, very advantageous to have a process for reforming light paraffins which exhibits high activity while producing a high yield of aromatics.